Many recipes call for a tomato of a certain size. How do you know the difference between small, medium and large tomatoes?
USDA requirements for measurements used by the Fresh Market tomato industry
**Size** **Size** No Pass Pass inches inches see note #1 see note #2 Small 2 1⁄8 inches 2 9⁄32 inches Medium 2¼ inches 2 17⁄32 inches Large 2½ inches 2 25⁄32 inches Extra Large 2¾ inches
note # 1. Will not pass through a round opening with this diameter when the tomato is held with its greatest diameter parallel to the plane of the opening.
note # 2. Will pass through a round opening of this size; the tomato may be held in any position for the test.
As SAJ14SAJ said, there's not really a true answer here. But insofar as there's anything at all official, it's probably the sizes the USDA uses for nutrition. They're still a bit arbitrary, but hey, everything will be.
Here's the full nutrition table on the USDA site. You can also get this kind of stuff just by googling "tomato nutrition" and looking in the knowledge panel on the right (there's a dropdown to pick serving sizes).
Obviously tomatoes do get plenty bigger than that, but I don't think most people write recipes expecting you to use a 5" beefsteak tomato.
It is going to be very context dependent. A small beefsteak tomato is probably larger than a very large Roma tomato.
That said, a recipe that is not precise about the type of tomato, nor the size of tomato is unlikely to be critically dependent on the exact quantity of tomato.
Still, one could estimate that a small tomato in the abstract is lime sized, a medium tomato lemon sized, and a large tomato orange to grapefruit sized. I realize this is no more concrete--there is no absolute standard.